Variable selectivity radio receiver



Patented July 25, 1939 PATENT 1 orr ce 2,167,400 vnnranm .samcrrvrrvaanro aroma John F. Farrington, Flushing, N. Y., assignor to HazeltineCorporation, a corporation of Del- Application January 21, 1937, SerialNo. 121.410

12 Claims. (or. 119-171) This invention relates to modulated-carriersignal receivers, and more particularly to adjustable band-pass selectorsystems suitable for controlling the selectivity or fidelity ofreproduc- 5 tion of such receivers automatically in accordance with thestrength of received signals.

In receivers including automatically adjustable band-pass selectorsystems, it is desirable that the width of the frequency band passed bythe selector be increased gradually with increasing signal inputs from aminimum, at signal strengths of the order of the sensitivity limit ofthe receiver, to a desired maximum, for signals of moderate intensityand that the width of theband l5 thereafter be maintained constant forfurther increases in signal strength. It is further desirable that thethreshold of signal input at which the band width of the selector beginsto expand from its minimum value be manually adjustable 20 so that, whenthe noise level is high, signals of greater strength are required toproduce band expansion. Also, it is desirable that means he provided formanually contracting the band width when receiving through stronginterfer- 25 ence.

Heretofore there have been proposed numerous band-pass selector systemsuseful in modulatedcarrier signal receivers and adjustable automaticallyin accordance with the intensity of received 30 signals to adjust theselectivity of the receiver,

thereby to enable the listener to procure the maximum fidelity ofreproduction consistent with conditions of reception obtaining. Amongsuch adjustable band-pass selectors, one type that has been found to beparticularly satisfactory, in that it provides symmetrical expansion ofthe pass band about the mean resonant frequency thereof, is of maximumsimplicity, requiring a minimum of additional tubes and circuitelements, and has .10 maximum flexibility of control, is that comprisinga pair of circuits resonant at frequencies within the pass band, adirectivecoupling means, such as a vacuum tube, coupling the circuits inone direction and a separate coupling means,

5 such as a second vacuum tube, coupling the circuits in the otherdirection, one or both of the coupling means being adjustable to varythe mutual impedance between the circuits, thereby to vary theselectivity of the receiver.

In band-pass selectors of the type described above, it has been foundthat a particularly Simple automatic control of the band width may beobtained by directly'controlling the responsiveness of one of thecoupling means, as, for exam- 5 pie, the forward coupling tube, inaccordance with received signal intensities and indirectly controllingthe responsiveness of the other coupling means in accordance with theresponse of the first coupling means, but in an opposite sense. However,the selectivity characteristics of such 6 band-pass selectors dependupon the joint efiect of both of the coupling means. With the circuitconstants and tube characteristics ordinarily encountered in band-passselectors of conventional receivers, the joint effect of the forward and10' backward coupling means, controlled as described above, oftenreaches a maximum value within the range of the automatic controllingeflect utilized as the direct control of the first coupling means sothat, for further variations of the direct control beyond thatcorresponding to the maximum joint eifect,- the selectivitycharacteristics are varied in a reverse sense; that is, the band passedby the selector is actually contracted, which is I opposite to theresult desired.

In other automatically adjustable band-pass selectors, on the otherhand, the band width tends to expand indefinitely in accordance-withincreasing signal inputs, resulting in excessive band widths which makethe receiver vulnerable to interference without correspondingimprovement in the fidelity of reproduction once the desired maximumband expansion has been exceeded.

It is an-object of the invention, therefore, to

provide, in a modulated-carrier signal receiver, an improvedautomatically adjustable band-pass selector system which will overcomethe abovementioned disadvantages of arrangements of the prior art andwhich will be simple in construction and eflicient and flexible inoperation and control.

It is a further object of the invention to provide, in amodulated-carrier signal receiver, an automatically adjustable band-passselector sys- 40 tern by means of which the selectivity of the receiveris varied-in accordance with received signal inputs and in which theband width passed by the selector is limited to a predetermined maximumvalue for signal inputs of a, given amplitude and is maintained constantfor signal inputs of greater amplitude.

More specifically, it is an object of the invention to provide aband-pass selector system including a pair of circuits resonant atfrequencies within the band, directive coupling means coupling thecircuits in one direction and separate coupling means coupling thecircuits in the other direction and in which the joint effect of the twocoupling means reaches a maximum value within the range of control ofone or both of the coupling means, by means of which variations of theselectivity of the receiver in a reverse sense for variations in thedirect control of one of the coupling means beyond that corresponding tothe maximum value of their joint efiect are avoided.

It is a further object of the invention to provide, in amodulated-carrier signal receiver, a band-pass selector of the characterdescribed in which, for values of the direct controlling action beyondthat corresponding tothe maximum joint eflect of the forward andbackward coupling means, the.control on the directly controlled couplingmeans is maintained at a constant value corresponding to such maximumjoint effect.

In accordance with one embodiment of the invention, there is provided,in a modulated-carrier signal receiver, a band-pass selector systemincluding a pair of circuits resonant at frequencies within the passband of the selector, directive coupling means coupling the circuits inone direction, separate coupling means coupling the circuits in theother direction, control means for adjusting at least one of thecoupling means to vary the mutual impedance between the circuits,thereby to vary the selectivity of the receiver, and means for limitingthe action of the control means in altering the selectivitycharacteristics of the receiver to a predetermined value, which may beadjustable.

More specifically, the band-pass selector in accordance with theinvention comprises a vacuum tube repeater in each of .the couplingcircuits, the repeater tubes having a common cathode circuit including abiasing resistor, and means for applying an adjustable bias to theforward coupling tube to vary its space current and thereby indirectlyadjust the bias and-transconductance of the backward coupling tube. Insuch a system, the selectivity of the receiver varies as the product ofthe transconductances of the forward and backward coupling tubes. Thecharacteristics of the tubes and the constants of their associatedcircuits are generally such that the space current of the forwardcoupling tube approaches a small minimum value within the limits of theadjustable bias so that, for higher values of bias voltage, thetransconductance of the forward coupling tube continues to decreasewhile the transconductance of the backward coupling tube does notcorrespondingly increase. As a result, the product of thetransconductances of theaterward and backward coupling tubes goesthrough v a maximum value. There is, therefore. provided means forlimiting the bias applied to the forward coupling tube to a preselectedvalue equal to or less than that required to attain such maximum valueof the said product to prevent variation of the selectivity of thereceiver in a reverse sense for values of said adjustable bias in excessof the preselected value. There may be provided also manually adjustablemeans for'increasing the negative bias potential applied to the backwardcoupling tube to maintain its transconductance at zero and the selectorband with at minimum for signal levels and resultant variable controlbias potentials greater than the threshold value normally efiective toinitiate expansion of the band width of the selector, for receivinthrough particularly high noise levels.

For a better understanding of the invention, together with other andfurther objects thereof, reference is had to the following specificationtaken in connection with the accompanying drawing and its scope will bepointed out in the appended claims.

In the drawing, Fig. l is a schematic circuit diagram of a completesuperheterodyne receiver employing an automatically adjustable band-passselector including means for limiting'the maximum band width of theselector in accordance with the invention; Fig. 2 is a circuit diagram,partially schematic, of a complete superheterodyne receiver embodying aparticular form of automatically adjustableband-pass selector sys- Item, shown in detail, together with means for limiting the maximum bandwidth of the selector; Fig. 3 is a simplified circuit diagram of theband-pass selector of Fig. 2 and its control cir cuit to aid in theunderstanding of the invention; while Fig. 4 is a graphillustratingoperating characteristics of the control systems of Figs. 1

and 2.

Referring now more particularly toFig. 1 of the drawing, there is shownschematically a superheterodyne receivenincluding, an adjustableband-pass selector and means for limiting the maximum band width passedthereby. In general, the receiver includes a tunable radio-ire quencyselector and amplifier I having its input circuit connected to anantenna-ground circuit 2, and its output circuit connected to a tunablefrequency changer 3, the output circuit of which is, in turn, connectedto an intermediate-frequenc-y selector and amplifier d. The selector andamplifier l comprises an adjustable bandpass selector of any suitabletype well known in the'art. Connected to the output circuit of theselector and amplifier 4 in cascade, in the order 'one or more of thetubes of the unit 4 by way of connection 9 and a high resistance elementH). In shunt with the source of A. V. C. potential in the controlconnection to the unit 4 is a unilaterally conductive device, such as adiode II in series with a source of adjustable negative potential,suchas a voltage divider l2 connected across a battery l3. The diode His connected with such polarity that it is non-conductive except whenthe negative A. V. C. bias exceeds that derived from the voltage divider[2.

The general operation of the'receiver justdescribed is well understoodin the art and a detailed description thereof is, therefore,unnecessary. 'In brief,-however, signals interceptedby the antenna 2 areselected and amplified in the radio-frequency amplifier I and aretransmitted to the tunable frequency changer 3, wherein they areconverted into intermediate-frequency signals in a well-known manner.The intermediatefrequency signals are selected and amplified in theintermediate-frequency amplifier 4 and passed to the detector 5, whereinthe audio frequencies of modulation are derived. The audiofrequencysignal is further amplified in the amplifier 6 and supplied in the usualmanner to the loud-speaker l for reproduction. The unidirectionalcomponent of the output of the A. V. C.

An automatic amplification con- Luc rectifier and detector 5 isappliedby way of the A. V. C. connection to one or more of the tubes of theradio-frequency amplifier I and the tunable frequency changer 3 throughthe connection 8, thereby maintaining the amplitude of the signal inputto the detector 5 within anarrow range for a widerange of receivedsignal amplitudes.

The circuit constants and tube characteristics of the adjustableselector unit 4 are so chosen that, for signal inputs of less than apredetermined amplitude usually near the sensitivity limit of thereceiver, the width of the frequency band passed by the unit 4 is aminimum, ensuring sub stantial freedom from adjacent signal interferenceand interchannel noise, but at a sacrifice in the fidelity ofreproduction. With increasing signal inputs, the signal-derived biasapplied to the unit 4 through the connection 9, effects a gradualexpansion of the frequency band passed by the unit 4, improving thefidelity of reproduction of the system. This expansion continues until,for signal inputs of moderate amplitude, the signal-derived bias exceedsthat derived from the voltage divider I2 and the diode H becomesconductive. Under this condition the signalderived bias is partiallydissipated in the resistor I0 and the bias applied to the unit 4 isthereafter maintained approximately constant, limiting the maximumexpansion of the frequency band passed by unit 4 to a desired value andthereafter maintaining such band of substantially constant width forfurther increases in signal input amplitudes. This limiting of .the bandexpansion is obtained without affecting the relation between theexpansion-control bias and theamplitude of the signal input for signalinputs of lower amplitude," that is, without limiting the normalsensitivity of the expansion control. Because of the high resistance ofthe element 10, there is no appreciable reaction back on the main A. V.C. circuit 8 to the units 1 and 3. By adjusting, the voltage divider I2,the value of the signalinput at which adjustment of the band passed byunit 4 is arrested may be adjusted as desired.

In Fig. 4 are shown curves representing band width-automaticcontrol-bias characteristics of typical automatically adjustableband-pass selectors, Curve A is typical of a selector in which the bandwidth increases continuously with increasing automatic control bias.Curve B is a characteristic of 'a type of automatically adjustableband-pass selector in which the band width increases from a minimum to amaximum with increasing automatic control bias and theredesired maximumband width. For example, if

the desired maximum'band width is represented by the ordinate C of Fig.4, further expansion or subsequent contraction can be prevented byapplying to the limiting diode II from the voltage divider 12 anegative-bias potential equal to the value D of Fig; 4. This preventsoperation of the diode ll until the automatic control bias reaches thevalue D, thus procuring gradual expansion of the band width withincreasing signal inputs up to this point and, as explained above,thereafter maintaining the bias to the unit 4 and the band width passedthereby substantially constant for'further increases in signal inputamplitude. The maximum band width of the selector is preferably limitedto a predetermined value corresponding to the usual modulation bandwidth of received signals, that is, to a band width sufllciently wide totranslate all the sideband frequencies of any received signal.

The application of the present invention to a particular form ofautomatically adjustable band-pass selector embodied in asuperheterodyne receiver is shown in detail in Fig. 2, in whichconventional elements corresponding to those of the system of Fig. 1 areindicated by like reference numerals. This system is of the same generaltype, involving the same general principles of operation, as that ofFig. 1, so that adescription-thereof need not be repeated. In Fig. 2 theintermediate-frequency amplifier and adjustable selector 4 and thesignal detector and A. V. C. rectifier 5 are shown in detail, the lattercomprising a conventional arrangement of a diode I4 having as its loadcircuit series-connected resistors l5 and I6 by-passed by condensers l1and I8, respectively, the A. V. C. connection 8 including suitablefilters comprising series resistors 23 and shunt condensers 24. Theaudiofrequency voltages applied to the amplifier 6 are derived from avoltage divider 19 coupled to the load circuit of the detector i4through a coupling condenser 20.

Referring now more particularly to the iritermediate-frequency amplifierand selector system 4 of Fig. 2, this system includes a pair of circuits25 and 26 tuned to the desired intermediate frequency and coupled withsomewhat less than optimum coupling, the circuit 25 being connected totheoutput of the frequency changer 3. Included also in the selectorsystem 4 is a pair of circuits 21 and 28, also tuned to the desiredintermediate frequency and coupled with somewhat .less than optimumcoupling, the circuit 28 being connected to the A. V. C. rectifier anddetector l4. Preferably, loading resistors 29 and 30 are connectedacross the circuits 25 and 28, respectively, to flatten the resonantcharacteristics of the system when adjusted for selectivity less thanminimum, as described hereinafter. The circuits 26 and 21 are coupled bya vacuum tube 3|, illustrated as of the pentode type, provided withcathodebiasing resistors 32 and 33 by-passed by condensers 34 and 35,respectively. Suitable operating potentials are applied to the screenand anode of the tube 31 from the sources indicated as +Sc and +8,respectively.

In order automatically to adjust the width of the frequency band passedby the system 4, the tuned circuits 26 and 21 are also coupled in areverse direction by a unidirectional coupling means, such as a vacuumtube, which may be of the triode type, as shown, having a relativelysharp cutoff characteristic. The input circuit of the. triode 31 iscoupled to the circuit 21 by means of a winding 38 coupled to theinductance element of that circuit and a blocking condenser 39. Theoutput circuit of the tube 31 is coupled to the circuit 26 by means of awinding 40 coupled to the inductance element of that circuit and abypass condenser Suitable anode potential is applied to the tube 31 froma source indicated as +13 through an isolating resistor 42. In order toneutralize'any capacitive forward coupling between the circuits 26 and21 through the interelectrode capacitance of the tube 3|, there isproprovides a capacitive feedback of the proper phase relation toneutralize the incidental capacitive forward coupling. With the use of atriode the interelectrode capacitance thereof is sumcient to accomplishthis neutralizing function so that a separate physical condenser 33 maybe omitted.

It will be noted that the biasing resistors 32 and '33 are included inthe cathode circuit of the feed-back tube and the values of theseresistors are so selected that the bias voltage developed thereacross,for signal inputs near or below the sensitivity limit of the receiver,biases the tube 3'?! to cutofi. The direct application of the A. V. C.bias to the forward coupling tube 3i effects a variation in the spacecurrent of this tube with signal intensity and a corresponding variationin the voltage drop across resistors 32 and 33, which voltage determinesthe bias on the backward coupling tube 31 and thus its transconductance.That is, the transconductance of the backward coupling tube iscontrolled indirectly in accordance with received signal intensities. Asdis= cussed above, however, in coupling systems of the type described,the selectivity characteristic is dependent upon the product of thetransconductances of the tubes 3i and 37, while the characteristics ofthe tubes 3! and 37, in connection with their associated circuits havingelectrical constants to satisfy the other requirements of the receiver,are such that the product of their transconductances usually reaches amaximum value for values of A. V. C. bias substantially less than themaximum normally used for controlling the gain of the receiver. That is,when the A. V. C. bias voltage exceeds that value corresponding to themaximum product of the transconductances of the tubes 3i and 37, furtherincreases in A. V. C. bias voltage corresponding to received signalinputs of larger amplitude, result in an increase in the selectivity ofthe receiver, which is the opposite of .the desired relationship.

In order to prevent this variation of the selectivity of the receiver ina reverse sense, there is provided an auxiliary control circuitconnected in parallel with that portion of the'automatic amplificationcontrol circuit of the tube 3| including the source of A. V. C. biaspotential derived from a load resistor l6 of the detector M, thisauxiliary circuit being effective to limit the bias potential applied tothe tube 3! to a. predetermined value for all values of A. V. C. biasvoltage ex-, ceeding said predetermined value. This auxiliary controlcircuit comprises a connection M to the junction'of the resistor it anda filter comprising resistor t5 by-pd ed by a. condenser eta, thecathode-anode circuit of a diode limiter tube 436, which may be includedwithin the same envelope as the rectifier N, the lower portion of thevoltage divider 47 between the adjustable tap Ma and ground, whichvoltage divider is energized from a source of constant-bias voltage suchas a battery 48, and, through the ground connection, the cathode-biasingresistor 33 to the cathode of the A. V. C. rectifier it.

The operation of the adjustable band-pass selector system 4 may beexplained by considering the flow of signal energy around the loopcomprising the input circuit 26, the forward coupling tube 3 I, theoutput circuit 21, and the backward coupling tube 37 having its inputcircuit coupled to the circuit 21 and its output circuit coupled to thecircuit 26. Considering the voltages induced in the circuit 26 from-thecircuit 25 as a reference, it will be apparent that alternating voltagesappearingacross the resonant circuit 21 are substantially reversed inphase with respect to those across the circuit 26 at frequencies in thevicinity of the resonant frequency of these circuits, at which frequencythese circuits are of high impedance and are substantially resistive.The feed-back voltages are reversed a second time in the backward pathin the tube 3?,

while a third reversal is secured in the coupling between the circuit 27and the winding 38, or between the winding 6t and the circuit 26 so thatvoltages induced across the input circuit 26 through this backwardcoupling path are, at the frequencies indicated, substantially reversedin phase with respect to the input voltages directly induced across thiscircuit from the primary circuit 25, and the system is degenerative tothe maximum degree at the intermediate-carrier frequency.

At frequencies substantially above the resonant frequency of thecircuits 26 and 21, these circuits are capacitively reactive towardvoltages thereacross so that the voltages at these frequencies acrossthe circuit 27 lag behind the input voltages by phase angles approaching90 degrees as a limit. The feedback voltages at these frequenciesdeveloped across the circuit 26 are similarly retarded by an additionalangle also approaching 90 degrees as a limit, so that the feed-backvoltages are nearly in phase with the input voltages at thesefrequencies, and the coupling system is regenerative.

At frequencies below the resonant frequency 01 the circuits 26 and 21',these circuits are inductively reactive toward voltages thereacross anda similar phase shift occurs but in the opposite Gil pedances of thecircuits 26 and 2'! are much less than at resonance, the transmissionemciency of the amplifier stage 3| being reduced, and the amplitude ofthe feed-back voltages being further reduced so that, while the systemis regenerative, it is entirely stable in operation. At frequenciesintermediate the limiting frequencies justdescribed, the feed-backvoltages, have intermediate phase angles with respect to the inputvoltages, and the feed-back characteristic of the system thus has agradual transition from degeneration at resonance to regeneration atfrequencies substantially displaced from resonance. Hence, the

resultant reduction in amplitude of the input voltages at frequenciesnear the intermediatecarrierfrequency and increase in amplitude of thevoltages at frequencies substantially above and below theintermediate-carrier frequency impart to the system a band-passfrequency characteristic like that of over-optimum coupled double-tunedcircuits.

By adjusting the forward and backward coupling reactions between theresonant input and output circuits, the shape and width of the bandpasscharacteristic may be controlled as desired. Such coupling control isprocured, in accordance with the present invention, by applying the am-,

plification control bias derived from the A. V. C. rectifier l4 directlyto the control grid of the forward coupling tube 3|, this biasincreasing with increaslng amplitudes of signal input to the v receiverand decreasing the transconductance and space current of the tube 3|,thus correspondingly decreasing the bias voltages across the resistors32 and 33. The latter resistors are included in the cathode circuit ofthe backward coupling tube 31, so that its grid'becomes less negativewith respect to its cathode, thereby increasing its transconductance.

Since, as explained above, the selectivity of the system is determinedby the product of the transductance of the forward coupling tube 3i, itis necessary to increase the transconductance of the backward couplingtube 31 to an even greater extent, so that it is necessary that the tube31 have a steeper grid voltage-transoonductance most easily secured byutilizing as the tube 31 a triode having a sharp cutoff characteristicand operating it in the vicinity of cutoff. While a triode used in sucha manner might introduce some distortion, this ismade negligible bycoupling the feed-back winding 38 very loosely to the circuit 21 so thatonly a small signal input voltage is appliedto the grid of the tube 31.

In the arrangement described above, in which the selectivity isdependent upon the product of the transconductances of the forward andbackward coupling tubes and thus is decreased with increasingamplification control bias corresponding to increasing received signalintensities with-- in a given range,'the usual range of amplificationcontrol bias is so wide that the product.of the transconductances of theforward and backward coupling tubes may reach a maximum and decreasewithin this wide range of bias voltages,

as explained above in connection with Fig. 4. In order to prevent suchoperation, the auxiliary control circuit described above is utilized. Tothis end, the automatic amplification control circuit is returned to thepositive terminal of the resistor 33 in the common cathode circuit ofthe tubes 3i and 31, the resistor 32 being provided to ensure propernormal grid bias to the tubes 3| and 31. The auxiliary control circuit,including the connection 44 to the automatic amplification controlcircuit at the junction of resistors l0 and 45 and the diode 46, iscompleted by an adjustable tap 41a. of the voltage divider 41, oneterminal of which is grounded. This auxiliary circuit has been redrawnin simplified form in Fig. 3, corresponding elements being identified bythe same refercnce'numerals, and all elements not essential to thisfeature of the invention being omitted for the sake of clarity.

Neglecting the ground connection. which is immaterial insofar as thisportion of the circuit is concerned, it will be seen that, in theauxiliary circuit, the amplification control bias across the V resistorI6 is opposed to the bias across the cathode resistor 33 and that acrossthe. portion of the voltage divider. 41 included in this circuit,through the diode 46. This diode is connected with such polarity that,for small values of amplification control bias across the resistor I6,it is nonconductive and the system operates as described above. As theamplification control bias rises to a predetermined value slightlyexceeding the sum of thevoltages across the resistor 33 and theeffective portion of the voltage divider 41, the diode 46 becomesconductive and completes a load. circuit for the resistor 16 throughresistor 10,;- the voltage drop across resistor 33 also remainsconstant. In brief, further increases 'in received signal intensity andamplification control bias across resistor l3 eifect no further increasein the bias voltages applied to the grids of the tubes 3i and 31 and theselectivity of the system is maintained constant, avoiding the tendencytowards reversal described above. By properly selecting the circuitconstants, the operating point at which the diode 46 becomes conductivemay be determined to cor-- respond to the maximum product of thetransconductances of the tubes 3! and 31, that is, the minimumselectivity of the receiver. By adjusting the tap 410, the value ofsignal intensity necessary to overcome the initial cutoff bias of thetube 31 may be adjusted to adjust the delay in operation of theautomatic selectivity'control. Adjustment of the tap 41a also adjuststhe value of the A. V. C. bias applied to the forward coupling tube 3|necessary to procure maximum band width to such an extent as tocompensate for the effect of the increased delay bias upon the minimumselectivity.

While the invention is applicable to a wide variety of band-passselectors having a" wide variety of selectivity characteristics, therefollow specifications of a particular band-pass selector havingautomatically adjustable selectivity characteristics which areparticulary satisfactory:

Forward co'uplingtube 3|, type 6K7 pentode. Backward coupling tube 31,type 6C5 triode. Intermediate frequency, 465 kilocycles.

Coupling transformers:

windings 25 and 28, L=1.2 mh. Universal coil of Litz'wire. 1%" wide and,4." high,

wound on 7 diameter polyiron core. 3

wound on outside of secondary coil and in r.

same direction. (Finish of coil connects to plate of control tube.)

Feed-back winding 38, 4 turns No. 38 wound on outside of primary and insame direction. (Start of coil connects to grid of control tube.)

Resistor Iii, 1 megohm. Resistor ,i5, 50,000 ohms.

Resistor i8, 250,000 ohms.

Resistor 29, 0.125 me'gohm.

Resistor 30, 1.0 megohm. N

No -signal voltage across 33, +6.5 volts. No-signal voltage across32+33, +9.5 volts. Voltage of tap 41a, 0 to '4 volts.

,.While there has been described what is at present considered to be thepreferred embodiment of this invention, it will be obvious to thoseskilled in the art that various changes and modifications may be madetherein without departing from the invention, and it is, therefore,aimed in the appended claims to cover all such changes and modificationsas fall within the true spirit and scope of the invention.

" What is claimed is:

1. In a modulated-carrier signal receiver including means for developinga signal-derived control bias, an adjustable bias controlled bandpassselector stage, means for applying said signal-derived bias to saidstage to controlthe band 1 width of said selector, and means forlimiting the maximum amplitude of said signal-derived bias applied tosaid stage, thereby to limit the maximum band width of saidselector to apredeter -mined value corresponding to the usual band in one direction,separate coupling means coupling said circuits in the other direction,said directive coupling means being adjustable to vary the mutualimpedance between said circuits in the direction of the directivecoupling thereby to vary the selectivity of the receiver, means foradjusting said directive coupling means in accordance with the action ofsaid gain control system, and means for limiting the maximum action ofsaid gain control system on said adjusting means thereby to limit themaximum band width of said selector system to a predetermined valuecorresponding to the usual modulation band width of received signals.

3. In a; modulated-carrier signal receiver including an automatic gaincontrol system, a

band-pass selector system including a pair of circuits resonant atfrequencies within said band,

directive coupling means coupling said circuits in one direction,separate coupling means coupling said circuits in the other direction,said directive coupling means being adjustable to vary the mutualimpedance between said circuits in the direction of the directivecoupling thereby to vary the selectivity of the receiver, means foradjusting said directive coupling means in accordance with the action ofsaid gain control system, means for limiting the maximum action of saidgain control, and means, efiective when the action of said gain controlsystem on said adjusting means exceeds a predetermined value, foreffecting a fixed adjustment of said directive coupling thereby to limitthe maximum band width of said selector system to a. predetermined valuecorrespondingv to the usual modulation band width of received signals.

4. In a modulated-carrier signal receiver ineluding a source ofautomatic amplification control bias, a band-pass selector systemincluding a pair -of circuits resonant at the mean frequency of saidband, directive coupling means coupling said circuits in one direction,separate coupling means coupling said circuits in the other direction,said directive coupling means being adjustable to vary the mutualimpedance between said circuits in the direction of the directivecoupling thereby to vary the selectivity of the receiver, controlmeansincluding a control circuit for utilizing said control bias foradjusting said directive coupling means thereby to vary the selectivityof the receiver in accordancetherewith, and means for limiting the biasutilized by said control circuit to a substantially fixed predeterminedvalue when said control bias exceeds said value thereby to limit themaximum band width of said'selector system to a predetermined valuecorresponding to the usual modulation band width of received signals.

5. In a modulated-carrier signal receiver including a source ofautomatic amplification control bias, a band-pass selector systemincluding a pair of circuits resonant at the mean frequency of saidband, directive coupling means coupling said circuits in one direction,separate coupling means coupling said circuits in the other direction,said directive. coupling means being adjustable to vary the mutualimpedance between said circuits in the direction of the directivecoupling thereby to vary the selectivity of the receiver, control meansincluding a control circult for utilizing said control bias foradjusting said directive coupling means thereby to vary the selectivityof-the receiver in accordance'therewith, and an auxiliary circuitincluding an auxiliary source of biasing potential and a.unidirectionally conductive device coupled with said control circuit,said auxiliary source and said control-bias source being efiectivelyconnected in series through said device, with opposite polarity, wherebythe bias utilized by said control circuit is limited to that of saidauxiliary source when tion, said directive coupling means being ad-'justable to vary the mutual impedance between said circuits in thedirection of the directive coupling thereby to vary the selectivity ofthe receiver, control means including a control circuit for utilizingsaid control bias for adjusting said directive coupling means thereby-tovary the selectivity of the receiver in accordance therewith, and anauxiliary circuit connected in parallel with 'a portion of said controlcircuit including said source of control bias, said auxiliary circuitincluding an auxiliary source of bias potential variable inversely inaccordance with said control bias potential, a second auxiliary sourceof constant-bias potential, and a diode rectifier, "said auxiliarysources being serially-connected to oppose said control source throughsaid rectifier, r whereby the bias utilized by said control circuit islimited to the sum of the potentials of said auxiliary sources when thepotential of said control-bias source exceeds the sum of the potentialsof said auxiliary sources and the maximum band width of said selector islimited to a predetermined value corresponding to the usual modulationband width of received signals.

'7. In a modulated-carrier signal receiver, a band-pass selector systemincluding a pair of circuits resonant at frequencies within said band, apair of directive coupling means individually coupling said circuit inforward and backward directions, said coupling means being adjustabletoprovide a mutual impedance between said circuits which is adjustable tovary the selectivity of the receiver, means for adjusting one of saidcoupling means, means responsive to adjustment of said one of saidcoupling means for adjusting the other of said coupling means in anopposite sense, whereby the selectivity of the receiver is dependent onthe joint eflect of both of said adjusting means, the response of saidlast-named means being such that said joint effect reaches a maximumwithin the limits of adjustment of said one of said coupling means, andmeans for limiting the adjustment'of said one of said coupling means toa value corresponding to the said maximum joint effect to preventvariation of the selectivity of thereceiver in a reverse sense.

8. In a modulated-carrier signal receiver, a

band-pass selector system including a pair of circuits resonant atfrequencies within said band, I a, first directive coupling meanscoupling said circuits in a forward direction, a second directivecoupling means coupling said circuits in a backward coupling means in anoppodte sense, where by the selectivity of the receiver isdependent uponthe product of the adjustments of said coupling means, the response, ofsaid backward "coupling adjusting means being such that said productreaches a maximum within the limits of adjustment of said forwardcoupling means, and means maintaining the adjustment of said .for-

ward coupling means constant at a value corresponding to the maximumvalue of said product for all adjustments of said forward cmiplingadljusting' neans corresponding to values of said product beyond saidmaximum, to prevent variation in 'the selectivity of the receiver in areverse sense.

9. In a modulated-carrier signal receiver, a

'band-pass selector system including 'a pair of circuits resonant at themean frequency of said coupling said circuits in the forward direction,means including a second vacuum tube repeater coupling said circuits ina backward direction,

said repeater tubes having a common cathode 4) circuit including abiasing resistor, means for applying an adjustable bias to saidforwardcoua pling tube to vary its transconductance and ftherebyindirectly adjusting the bias and trans-- conductance of said backwardcoupling tube, whereby the selectivity of the receiver varies as theproduct of said transconductances, the characteristics of said tubesbeing so related to the constants of their cathode-biasin circuit thatsaid product reaches a maximum within the lim- 50 its of said adjustablebias, and means for limiting the bias applied to said forward coupling.

tube to a preselected value to prevent variation of the selectivity ofthe receiver for values of said adjustable bias in excess of saidpreselected 55 value. j

10. In a modulated-carrier signal receiver, a

band-pass selector system including a pair of circuits resonant atfrequencies'within said band,

means including a vacuum tube repeater coum pling said circuits in theforward direction,

means including a second vacuum tube repeater coupling said circuits ina backward direction,

band, means including a vacuum tube repeater:

said product reaches a maximum within the limits of said adjustablebias, and means for limiting the bias applied to said forward couplingtube to ,a. preselected valtre corresponding to the maximum value ofsaid product to prevent 5 variation of the selectivity of'the receiverin a reverse sense for values of said adjustable bias in excess of saidpreselected value.

11. In a modulated-carrier signal receiver. .a band-pass selector systemincluding a pair of circuits resonant at frequencies within said band.means including a vacuum tube repeater coupling said circuits in theforward direction, means including a second vacuum tube repeatercoupling-said circuits in a backward direction 15 whereby theselectivity of the receiver varies as the product of thetransconductances of said tubes, said repeater tubes havinga commoncathode circuit including a biasing resistor, a source of. adjustablebias voltage, means for applying 20 anadjustable bias derived from saidsource to said forward coupling tube to vary itsspace current andthereby indirectly adjusting the bias and transconductance of saidbackward coupling ,tube, the characteristics oi'said tubes being so 25related to the constants of their cathode-biasing circuit that saidproduct reaches a maximum.

within the limits of said adjustable bias, and

a pair of circuits resonant at frequencies within said band, meansincluding a vacuum tube repeater coupling said circuits in the forwarddirection, means including a second vacuum tube repeater coupling saidcircuits in a backward direction whereby the-selectivity of the receivervaries as the product of the transconductances of said tubes, saidrepeater tubes having a common cathode circuit including a biasingresistor, means including a control circuit for applying a control-biasderived from said resistor to said forward coupling tube to vary itstransconductance inversely in accordance with received signal inputs andthereby indirectly to.vary the transconductance of said backward tube inan opposite sense, the transconductance characteristics of said tubesbeing so related to the con- ,stants of their cathode-biasing circuitthat said product reaches a maximum for an amplification control bias ofa'given value, and an auxiliary.

circuit connected in parallel with a portion of said control circuitincluding said source of control bias, said auxiliary circuit includingan auxiliary source of bias potential variable inversely in accordancewith said control-bias potential, a second auxiliary source of constant-5 bias potential and a diode rectifier, said auxiliary sources beingserially-connected to oppose said control source through said dioderectifier,

whereby the bias applied to said forward coupling tube by said controlcircuit is limitedto the sum of the biases of said auxiliary sourceswhenv the potential of said control-bias source exceeds the sum of thepotentials of said auxiliary sources, thereby to prevent variation inthe selectivity of the receiver when said control bias exceeds saidgiven value.

' JOHN F. FARRINGTON.

